1. Technical Field
The present invention relates to a parallel-serial converter for optical transmission, an optical transmission system, and an electronic apparatus.
2. Background Art
In recent years, higher data transmission speed between an LCD (Liquid Crystal Display) and an application processor is being demanded with higher definition of the LCD of the portable telephone. Furthermore, reduction in the number of wirings for data transmission is also being demanded with advancement in the thinning of the portable telephone. Based on such background, the serial transmission is beginning to be widely used as a data transmission method between the LCD and the application processor in place of the parallel transmission of the related art. However, in the conventional electrical wiring, there is a limit to increasing the transmission speed because smaller space of the wiring and electromagnetic interference (EMI) become significant. Thus, a method of connecting the LCD and the application processor with an optical transmission path such as a light waveguide and transmitting the data signal as an optical signal is considered.
The light waveguide has a dual structure including a center core called the core and a casing called the clad for covering the same, where the index of refraction of the core is higher than that of the clad. The optical signal that entered the core is propagated by repeating total reflection inside the core.
A schematic configuration of an optical transmission module including the optical transmission path will be described below using the drawing. FIG. 18 is a block diagram of a portion applied with the optical transmission module in a foldable portable telephone incorporating the optical transmission module.
An optical transmission module 100 includes a main control substrate (master side substrate) 20, and an application circuit substrate (slave side substrate) 30. The main control substrate 20 is mounted with a CPU 29. The application circuit substrate 30 is mounted with an LCD (Liquid Crystal Display), an LCD driver 39 for drive controlling the LCD, and various types of applications such as a camera module.
The main control substrate is connected with a light transmission processing section 2 including a light source drive circuit (light emission drive unit) and a light emitting unit (light emitting element; VCSEL (Vertical Cavity-Surface Emitting Laser). The slave side substrate 30 is connected with a light reception processing section 3 including a light receiving unit (light receiving element; PD (Photo-Diode)) and a reception (amplifier) IC. The optical transmission path 4 such as an optical fiber or a light waveguide having high bendability connects the light transmission processing section 2 with the light reception processing section 3 to transmit optical signals.
The mechanism of optical transmission in the optical transmission module 100 will now be briefly described. First, a light emission drive unit (driver) 22 drives the light emission of a light emitting unit 33 based on an electric signal input from the main control substrate 20 through an interface circuit (hereinafter referred to as I/F circuit) 21, so that the light emitting unit 23 irradiates a light incident surface of the optical transmission path 4 with light. The light applied on the light incident surface of the optical transmission path 4 is introduced into the optical transmission path 4, and exit from a light exit surface of the optical transmission path 4. The light exit from the light exit surface of the optical transmission path 4 is received by the light receiving unit 31, and converted to an electric signal after light reception is detected by a detection circuit 32. The converted electric signal is amplified to a desired value by an amplifier unit 33, and input to the LCD driver 39 or the like of the application circuit substrate 30 through an I/F circuit 34.
High speed and large capacity data transmission from the main control substrate mounted in the portable telephone to the application circuit substrate thus can be carried out by using such optical transmission module. The optical transmission module thus greatly excels as a data transmission module.
If the optical transmission module is applied to the electrical wiring I/F circuit for existing portable telephone as in the configuration described above, the low pass signal may not be easily transmitted due to the properties of the optical transmission module. In particular, when the continuing length of bits of the signal to be transmitted becomes long, the low pass properties of the transmitting signal extend. Thus, problems arise in introducing the optical transmission path as it is to the serial transmission by the electrical wiring of the related art.
For instance, Patent Document 1 discloses a configuration in which a coding unit for coding an output signal is arranged in the CPU 29 to add a coding function to prevent bit continuation of the transmitting signal (prevent transmission of low pass signal) in the optical transmission system. The 8B10B conversion is generally known for the coding function. The 8B10B conversion is an algorithm of data transmission coding in which the information of 8 bits are represented with a symbol (transmission character) of 10 bits. According to such 8B10B conversion, the value of “0” does not continue for a certain number or more in the transmitting signal.
However, the CPU 29 added with the coding function has a problem in that the power consumption, the size, and the cost increase. The configuration disclosed in Patent Document is thus not suited for practical use.
Furthermore, adopting the CPU 29 with no coding function to the optical transmission system is difficult because there is little IC commercial items having only the coding function. If in the IC commercial item having only the coding function the CPU 29 not added with the coding function is adopted in the optical transmission system, various problems such as increase in the power consumption of the optical communication IC, degradation in the signal waveform characteristics, and adjustment of sequence timing arise, and thus it is not suitable for practical use.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-230678